Q&A: practice questions with answers

Concept: Power $P=\frac{1}{f}$ with $f$ in metres; unit dioptre (D). Negative $f$ means a concave lens.

1. $f=-25\text{cm}=-0.25\text{m}$
2. $P=\frac{1}{-0.25}=-4\text{D}$

Trick: With $f$ in cm: $P=\frac{100}{f}=\frac{100}{-25}=-4\text{D}$.

Answer: (C) $-4$ D

Concept: Carbon atoms can link with other carbon atoms to form long chains, branched chains and rings. This self-linking property is called catenation.

1. Carbon is tetravalent and C–C bonds are very strong, so long stable chains form.
2. This is why carbon forms millions of compounds.
3. Polymerisation is joining of monomers; it is not the name of the self-linking property.

Yaad rakhein: Two special properties of carbon: catenation and tetravalency.

Answer: (A) Catenation

Concept: Mass is the quantity of matter in a body — constant everywhere. Weight is the force of attraction of the earth, $W=mg$, so it changes with $g$.

1. Option 1 is wrong: weight is a force, not power.
2. Option 3 is wrong: $W=m\times g$, not velocity $\times g$.
3. Option 4 is wrong: weight changes from place to place (e.g., on the moon it is about $\frac{1}{6}$).
4. Option 2 is correct: mass stays constant.

Yaad rakhein: Mass — scalar, kg, constant. Weight — force, N, varies with $g$.

Answer: (B) The mass of an object is constant and does not change from place to place.

Concept: Weight of a body on earth: $W=mg$.

1. $m=3\text{kg}$, $g=9.8{\text{m s}}^{-2}$
2. $W=3\times 9.8=29.4\text{N}$

Trick: $3\times 10=30$, so the answer must be a little less than $30$ → $29.4$.

Answer: (B) 29.4 N

Concept: Soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids, e.g. sodium stearate ${\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}$.

1. Detergents are sodium salts of long-chain sulphonic acids or ammonium salts — not carboxylic acids.
2. A micelle is the cluster that soap molecules form in water, not the compound itself.
3. So the given definition is of soap.

Yaad rakhein: Soap = $-\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}$ (carboxylate); detergent = $-\mathrm{S}{\mathrm{O}}_3\mathrm{N}\mathrm{a}$ (sulphonate).

Answer: (C) Soap

Concept: When an object is placed between the pole and focus of a concave mirror, it forms an enlarged, erect, virtual image.

1. A dentist holds the concave mirror close to the tooth (within $F$).
2. The tooth appears magnified and upright, so it can be examined clearly.
3. Convex mirror gives diminished images; plane mirror gives same-size images. 'Plano-convex' is a lens shape, not a mirror.

Yaad rakhein: Concave mirror uses: dentist's mirror, shaving mirror, torch/headlight reflector, solar furnace.

Answer: (C) a concave mirror

Concept: A colloid looks uniform but is actually a heterogeneous mixture; its particles (about $1$–$100\text{nm}$) are too small to see but bigger than solution particles.

1. Particles do not settle on standing — true property.
2. They pass through filter paper, so cannot be separated by filtration — true property.
3. Colloid is heterogeneous — true. Hence 'colloid is a homogeneous mixture' is NOT a property.

Yaad rakhein: Solution = homogeneous; colloid and suspension = heterogeneous. Colloids show the Tyndall effect.

Answer: (D) Colloid is a homogeneous mixture.

Concept: An ecosystem has biotic (living) components and abiotic (non-living, physical) components.

1. Temperature, rainfall, wind, soil and minerals are non-living factors → abiotic.
2. Plants, animals and microbes (producers, consumers, decomposers) are biotic.
3. Decomposers are living organisms, so that option does not fit.

Yaad rakhein: 'A' in abiotic = 'without life' → physical and chemical factors.

Answer: (B) abiotic components

Concept: Splitting of white light into its seven colours by a prism is called dispersion.

1. Isaac Newton first used a glass prism to obtain the spectrum of sunlight.
2. He placed a second, inverted prism and recombined the colours into white light, showing sunlight is made of seven colours.
3. C. V. Raman is known for the Raman effect (scattering), not for the prism spectrum.

Yaad rakhein: Spectrum order: VIBGYOR — violet bends the most, red the least.

Answer: (A) Sir Isaac Newton

Concept: Impure metals like copper, zinc, tin, nickel, silver and gold are purified by electrolytic refining.

1. Impure metal is made the anode; a thin strip of pure metal is the cathode.
2. A salt solution of the same metal is the electrolyte.
3. On passing current, pure metal deposits on the cathode; impurities settle as anode mud.
4. Oiling, painting and galvanisation only prevent rusting — they do not purify.

Yaad rakhein: Anode = impure, Cathode = pure. Galvanisation = zinc coating on iron.

Answer: (A) Electrolytic refining

Concept: Purity of gold is measured in carats; 24 carat is the maximum, i.e. pure gold.

1. 24-carat gold is pure but very soft, so it is not used for jewellery.
2. Jewellery gold is usually 22 carat: $22$ parts gold + $2$ parts copper or silver (an alloy).
3. Purity of 22 carat $=\frac{22}{24}\times 100\approx 91.7\%$.

Yaad rakhein: Carat value $÷24$ = fraction of gold.

Answer: (C) pure gold

Concept: Electrolysis of aqueous NaCl (brine) is the chlor-alkali process: $2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+\mathrm{C}{\mathrm{l}}_2+{\mathrm{H}}_2$.

1. Chlorine gas is released at the anode.
2. Hydrogen gas is released at the cathode.
3. Sodium hydroxide solution forms near the cathode.
4. Oxygen gas is not a product.

Yaad rakhein: Chlor ($\mathrm{C}{\mathrm{l}}_2$) + alkali ($\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}$) + ${\mathrm{H}}_2$ — name itself tells the products.

Answer: (D) Oxygen gas

Concept: For a mirror, magnification $m=-\frac{v}{u}$. With the sign convention, distances in front of the mirror are negative.

1. Object in front: $u=-20\text{cm}$; image on the same side (real): $v=-40\text{cm}$.
2. $m=-\frac{v}{u}=-\frac{-40}{-20}=-2$
3. Negative sign → real, inverted image, twice the size.

Trick: Real image of a mirror always has negative $m$; size ratio $=\frac{40}{20}=2$ → $-2$.

Answer: (D) −2.0

Concept: Kinetic energy $KE=\frac{1}{2}m{v}^2$.

1. $KE\propto m$ → directly proportional to mass.
2. $KE\propto v^2$ → directly proportional to the square of velocity.
3. E.g., doubling $v$ makes $KE$ four times.

Trick: Read the formula: $m$ and $v^2$ are both in the numerator → both 'directly proportional'.

Answer: (D) directly proportional to its mass and square of its velocity

Concept: At puberty, sex hormones bring changes; some are common to both sexes, some specific to girls or boys.

1. Girls: increase in breast size, start of menstruation.
2. Both sexes: oily skin, pimples, hair in armpits and genital area.
3. Cracking of voice (and facial hair) is a sign in boys, due to testosterone and growth of the larynx.

Yaad rakhein: Cracking voice, beard, moustache → boys only.

Answer: (A) Cracking of voice

Concept: Autotrophs (green plants) take in $\mathrm{C}{\mathrm{O}}_2$ and water from outside and, using sunlight and chlorophyll, convert them into carbohydrates — stored energy. This is photosynthesis.

1. Equation: $6\mathrm{C}{\mathrm{O}}_2+12{\mathrm{H}}_2\mathrm{O}\stackrel{\text{sunlight, chlorophyll}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}$
2. Glucose formed is stored as starch.
3. Digestion, circulation and movement do not convert light energy into chemical energy.

Yaad rakhein: Photo = light, synthesis = making — 'making food using light'.

Answer: (A) Photosynthesis

Concept: Without air resistance, all bodies fall with the same acceleration $g$, whatever their mass. Time to fall height $h$: $t=\sqrt{\frac{2h}{g}}$.

1. $t$ depends only on $h$ and $g$ — mass does not appear.
2. Both bodies fall from the same height, so $t_1=t_2$.
3. Ratio $t_1:t_2=1:1$.

Yaad rakhein: Galileo: a feather and a coin fall together in vacuum.

Answer: (C) 1:1

Concept: Movement of plant parts in response to gravity is geotropism (gravitropism).

1. Roots grow downward, towards gravity → positive geotropism.
2. Shoots grow upward, away from gravity → negative geotropism.
3. So both upward and downward growth are due to geotropism. (Phototropism = light, hydrotropism = water, chemotropism = chemicals, e.g. pollen tube.)

Yaad rakhein: Geo = earth: roots go to the earth, shoot goes away from it.

Answer: (A) Geotropism

Concept: $1\text{m/s}=\frac{3600}{1000}\text{km/h}=\frac{18}{5}\text{km/h}$.

1. $25\text{m/s}=25\times \frac{18}{5}\text{km/h}$
2. $=5\times 18=90\text{km/h}$

Trick: m/s → km/h: multiply by $3.6$: $25\times 3.6=90$.

Answer: (B) 90 km/h

Concept: Highly reactive metals like sodium and potassium react violently even with cold water.

1. $2\mathrm{N}\mathrm{a}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2$ (heat released; hydrogen may catch fire).
2. Aluminium, iron and zinc do not react with cold water; they react only with steam.
3. Sodium is therefore stored in kerosene.

Yaad rakhein: Cold water: K, Na (violent), Ca (less violent). Hot water: Mg. Steam: Al, Zn, Fe.

Answer: (D) Sodium